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KellsMac

Slip Casting - need help please

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Hi guys, I am new to slip casting and could use some help/advice from those of you with more experience.  I have been making plaster molds (with pottery plaster) and doing some basic slip casting at the community college where I take classes.  No one else at the studio has any experience with slip casting so Im mostly making things up as I go along and coming up with lots of ways that don't work.  So my main question at the moment involves making the slip.  I have tried premade slip from Laguna (sp?) and I have used dry clay (flint hills cone 10 grey stoneware and cone 10 domestic porcelain also from Flint Hills.  For both of the dry clays I have been adding water to the dry clay, letting it sit 12-24 hours, and then testing specific gravity and adding more dry clay to try to approach a specific gravity of 1.82.  When I get the specific gravity high enough I have been adding sodium silicate as a defloculant to get it to a pourable consistency.   

specific questions... 1)is this how you make your slip?

2) How thick is your slip before you add the defloculant? mine is pretty much like peanut butter.  You could make a squishy sculpture out of this stuff.  My instructor says he thinks it should be like heavy cream... but the spefiic gravity is no where near high enough if its still liquid before I add the defloculant. 

3) I usually give up on trying to get the specific gravity high enough around 1.60 ish when its still somewhat mixable/liquid-like and add the sodium silicate then.    I try to get the mix pourable... but as soon as it sits still for more than a minute or two it starts to thicken up again.

even with these issues my items have been coming through the bisque fine... I havnt actually glazed anything yet so I don't have final results.  Anyway.... would someone be willing to run me through their slip making process?  Ive tried looking for videos online and havnt had any success.  thanks so much for your time.

kel

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You need a slip hydrometer and a ford fluid cup for timing the fluidity.

If you are making only a few things just by ready made slip

If its hundreds you should make slip

I had a slip business many decades ago and had our formula made up dry by the ton-cone 10 porcelain slip

I may still have the formula on the wall of defloculants additions -but I can tell you it was Darvon that we added.

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I do a low-fire terra cotta slip, so I can't speak to your cone 10 slip, but I use both Sodium Silicate and Darvan.  Both are important to get the correct viscosity - if you use just one you will approach over-deflocculation. 

How long are you mixing for?  When I make a fresh batch of slip, I make sure it mixes for a minimum of 6 hrs before casting.  Any time after that, if I adjust deflocculants, a minimum of one hour before casting.  I know this is difficult if you don't have an industrial mixer though.  

Final thought: if your results are coming through bisque fine, maybe you don't have anything to worry about?  It's hard to give advice without final results.  

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